modalDialog: Do not sync display here

This gdk_display_sync() call was added in commit a40daa3c22 so the alt-f2
dialog is able to spawn commands that trigger grabs on startup (eg. xmag/
xkill).

This seems worthwhile to do only on the X11 backend, and handling it in
mutter backend code seems cleaner.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/317

js/ui/modalDialog.js

@@ -1,7 +1,6 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
 const Clutter = imports.gi.Clutter;
-const Gdk = imports.gi.Gdk;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Meta = imports.gi.Meta;
@@ -197,7 +196,6 @@ var ModalDialog = class {
         else
             this._savedKeyFocus = null;
         Main.popModal(this._group, timestamp);
-        Gdk.Display.get_default().sync();
         this._hasModal = false;
 
         if (!this._shellReactive)